1. Field of the Invention
The present invention relates to a dry presensitized plate and more particularly to a dry presensitized plate for use in making a lithographic printing plate requiring no dampening water during printing.
2. Description of the Prior Art
There have been proposed a various kind of dry presensitized plates (hereunder referred to as dry PS plate(s) for simplicity) for use in making lithographic printing plates requiring no dampening water during printing, which comprises a support having provided thereon, in order, a light-sensitive layer and a silicone rubber layer. In this respect, reference is made to, for example, Japanese Patent Publn. Nos. 44-23042; 46-16044; 51-17081; 54-26923 and 55-22781 and Japanese Patent Application (unexamined and opened to public inspection, hereinafter referred to as "OPI") No. 56-80046. Methods for making printing plates from the aforementioned dry PS plate are roughly divided into the following two groups; one of which comprises removing a silicone rubber layer in image areas (ie., exposed or nonexposed areas depending on the kind of the PS plate) by dissolving the underlying light-sensitive layer in the image areas with a developer to form images (eg. printing areas), see, for instance, Japanese Patent Publn. No. 46-16044; the other of which comprises depositing or applying a photoadhesive light-sensitive layer (in the case of positive-working PS plate) or a photoreleasable light-sensitive layer (in the case of negative-working PS plate) and selectively removing silicone rubber layer only in the areas causing no photoadhesion or those causing photorelease upon imagewise exposure by swelling (so-called method for selectively swelling and removing only the silicone rubber layer on the light-sensitive layer), see, for instance, Japanese Patent Publn. No. 54-26923 and Japanese Patent Appln. (OPI) No. 56-80046.
In any event, it is necessary to remove the silicone rubber layer with a developer in these methods. This makes the plate making process more complicated and further gives rise to a variety of problems of hygiene, public pollution and safety, depending on agents used for development. Therefore, such methods for making printing plate including a procedure for removing a silicone rubber layer with a developer are not preferred.
On the contrary, a dry PS plate which permits the plate making without using a liquid developer is disclosed in Japanese Patent Publn. No. 56-41105 and U.S. Pat. No. 4,271,255. These dry PS plates are provided with a cover sheet on the silicone rubber layer directly or through an adhesive layer. This cover sheet serves to enhance the adhesive force between the cover sheet and the silicone rubber layer upon exposure to light to an extent that the adhesive force therebetween is superior to that between the silicone rubber layer and the underlying layer thereof and thus the silicone rubber layer in exposed areas is simultaneously removed when the cover sheet is removed. However, this kind of dry PS plate suffers a variety of disadvantages such that the quality of images is lowered because of such removing treatment, that it takes much time and requires much labor to treat the removed cover sheet and so forth.